Chronic obstructive pulmonary disease (COPD) is among the most important chronic diseases of adults. It is the fourth leading cause of death in the U.S. and affects more than 10% of the U.S. population over age 55. COPD accounts for more than 25,000 discharges annually from VA medical centers. Cigarette smoking is the most important known modifiable cause of COPD, yet only 15-20% of cigarette smokers develop clinically significant COPD. We therefore hypothesize that genetic mechanisms determine susceptibility to the development of COPD. The only genetic abnormality known to cause COPD is deficiency of the serine protease inhibitor 1-protease inhibitor, which causes premature development of emphysema, although it is implicated in fewer than 2% of COPD cases. Several other genetic variants have been proposed as candidate causes of COPD; however, these have been identified on the basis of association studies in unrelated subjects, which have considerable risk of ascertainment bias. In the present study, a sub-pair linkage approach will be utilized in COPD patients and their smoking siblings to identify genes which determine the risk of developing COPD. The following are the short-term objectives of this study: